Programmable gain amplifier with glitch minimization

ABSTRACT

A programable gain amplifier (PGA) has an amplifier and a variable resistor that is connected to the output of the amplifier. The variable resistor includes a resistor that is connected to a reference voltage and multiple parallel taps that tap off the resistor. A two-stage switch network having fine stage switches and coarse stage switches connects the resistor taps to an output node of the PGA. The taps and corresponding fine stage switches are arranged into two or more groups, where each group has n-number of fine stage switches and corresponding taps. One terminal of each fine stage switch is connected to the corresponding resistor tap, and the other terminal is connected to an output terminal for the corresponding group. The coarse stage switches select from among the groups of fine stage switches, and connect to the output of the PGA. During operation, one selected tap is connected to the output of the PGA by closing the appropriate fine stage switch and coarse stage switch, where the selected tap defines a selected group ofthe fine stage switches. Additionally, one fine stage switch is closed in each of the non-selected groups of fine stage switches. In one embodiment, the location of the closed switches in the non-selected groups is the mirror image of the location in an adjacent group. This reduces the transient voltages that occur when tap selection changes from one group to another.

This application is a continuation of U.S. application Ser. No.09/969,793, filed on Oct. 4, 2001, which claims the benefit of U.S.Provisional Application No. 60/286,534, filed on Apr. 27, 2001, both ofwhich are incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to automatic gain control in areceiver, and more specifically to a programmable gain amplifier (PGA)that performs automatic gain control while minimizing transient voltagesduring tap changes.

2. Background Art

In electronic communications, electromagnetic signals carry informationbetween two nodes over a connecting medium. Exemplary media includecable, optical fiber, public airways, etc. The signal strength at thereceiving node varies depending on the distance between the nodes andchanges in the condition of the medium. For example, the signal strengthtypically decreases with increasing distance between the two nodes.Furthermore, even if the distance is fixed, physical variations in themedium over time can affect signal strength. For example, in a cablesystem, different cables can have different attenuation constants. Also,increased moisture content in a cable line, or in the public airways canreduce signal strength at the receiver. Finally, variations intransmitter output power will also affect signal strength at thereceiver.

An automatic gain control (AGC) circuit and a programmable gainamplifier (PGA) are often used at the receiver input to compensate forvariations of received signal strength. More specifically, the AGCcircuit adjusts the gain setting of the PGA to maintain the signalstrength within a desired operating range. If the received signalstrength is too high, then the AGC lowers the gain setting of the PGA.If the received signal strength is too low, then the AGC raises the gainsetting of the PGA. When the AGC is changing the gain of the PGA, thereis a possibility of introducing a glitch in the system. The glitchmanifests itself as an unwanted transient voltage that can cause avoltage detection error if the transient voltage does not settle withinspecified time period, for example one clock cycle.

What is needed is PGA configuration that quickly settles any transientvoltage caused by changing gain settings. Furthermore, the PGAconfiguration should have sufficient operating bandwidth.

BRIEF SUMMARY OF THE INVENTION

The present invention is a programable gain amplifier (PGA) having anamplifier and a variable resistor that is connected to the output of theamplifier. The variable resistor includes a resistor that is connectedto a ground or reference voltage, and multiple parallel taps that tapoff the resistor. Additionally, the PGA includes a two-stage switchnetwork having fine stage switches and coarse stage switches thatconnect the resistor taps to an output node ofthe PGA. The taps andcorresponding fine stage switches are arranged into two or more groups,where each group has n-fine stage switches and corresponding taps. Oneterminal of each fine stage switch is connected to the correspondingresistor tap, and the other terminal is connected to an output terminalfor the corresponding group. The coarse stage switches are connected tocorresponding group output terminals and select a group of fine stageswitches to connect to the output of the PGA.

During operation, one tap is selected to be connected to the output ofthe PGA by closing the appropriate fine stage switch and coarse stageswitch, where the selected tap defines a selected group of the finestage switches. Additionally, one fine stage switch is closed in each ofthe non-selected groups of fine stage switches. In one embodiment, thelocation of the closed switches in the non-selected groups is the mirrorimage of the location in an adjacent group. In Other words, if them^(th) fine stage switch is closed in a first group of fine stageswitches, then the [(n+1)−m]^(th) fine stage switch is closed a secondgroup of fine stage switches that is adjacent to the first group of finestage switches, assuming the fine stage switches are indexed from 1-to-nin each group. This reduces the transient voltages that occur when tapselection changes from one group to another.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates an exemplary receiver environment having aprogramable gain amplifier (PGA);

FIG. 2 illustrates a conventional PGA 200;

FIG. 3A illustrates a PGA 300 with a two stage switch configurationaccording to embodiments of the invention;

FIG. 3B illustrates a parasitic capacitance associated with the PGA 300;

FIGS. 4A-4B illustrate example two stage switch PGA configurations withat least one switch turned on in each group of fine stage switches;

FIGS. 5A-5E illustrate example two stage switch PGA configurations withone or more switches turned on in each group of fine stage switches,according to embodiments of the present invention;

FIG. 6 illustrates the 3 dB cutoff frequency vs. PGA gain setting for aPGA that is operated according to embodiments of the present invention;and

FIG. 7 illustrates a flowchart 700 of that describes the operating theswitches in the PGA according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION 1. Example Receiver Application

Before describing the invention in detail, it is useful to describe anexample receiver environment for the invention. The programable gainamplifier (PGA) invention is not limited to the receiver environmentthat is described herein, as the PGA invention is applicable to otherreceiver and non-receiver applications as will be understood to thoseskilled in the relevant arts based on the discussions given herein.

FIG. 1 illustrates an environment 100 having a medium 102, and areceiver 106 that receives a communications signal 104 carried by themedium 102. The receiver 106 includes a programable gain amplifier (PGA)108, an analog-to-digital converter (ADC) 110, a digital signalprocessor (DSP) 112, and an automatic gain control (AGC) 116. Thereceiver 106 receives the communications signal 104 from the medium 102,and extracts an information signal 114. More specifically, the PGA 108receives the communications signal 104 and variable amplifies the signal104 as determined by the AGC 116 to generate a PGA output signal 109.The ADC 110 converts the PGA output 109 to a digital signal 111. The DSP112 processes the digital signal 111 to generate the information signal114. For example, the DSP 112 examines the voltage of the digital signal111 to determine if the voltage represents a “0” or a “1” in order toretrieve the information signal 114. The DSP 112 may also perform acyclic redundancy check (CRC) on the bit stream of the digital signal111 to determine if there have been any errors that were introducedduring transmission.

The signal strength ofthe input signal 104 can vary based on thephysical characteristics of the medium 102. In cable systems forexample, a longer cable will typically have more attenuation than ashorter cable, thereby affecting the signal strength ofthe signal 104.In order to compensate, the AGC 116 detects the signal strength of thedigital signal 111 and adjusts the gain settings of the PGA 108 usingAGC control signal 117 to maintain a relatively constant signalstrength. For example, if the signal strength of the digital signal 111is too weak, then the AGC 116 increases the gain setting ofthe PGA 108to increase the signal strength. Alternatively, if the signal strengthof the digital signal 111 is too strong, then the AGC 116 decreases thegain setting of the PGA 108 to decrease the signal strength.

Without AGC compensation, these signal strength variations wouldadversely affect the accuracy of the information signal 114. Forexample, if the received signal 104 is too strong, then the ADC 110 canbe saturated. Conversely, if the digital signal 111 is too weak, falsepositives can be generated during the CRC error check that is performedby the DSP 112.

2. Conventional PGA

FIG. 2 illustrates a conventional PGA 200 that includes an amplifier 202and a variable resistor 210 that is connected to the output of theamplifier 202. The amplifier 202 can be any type amplifier including abuffer amplifier. The variable resistor 210 includes a resistor 204 thatconnects the output of the amplifier 202 to ground or a referencevoltage. The resistor 204 has multiple parallel taps 206 a-n that tapoff the resistor 204 (e.g. resistor ladder) to a common node 214, whichis the output of the PGA 200. Switches 208 a-n connect the correspondingtaps 206 a-n to the common node 214. The switches 208 are controlled bya control signal 212, such as the AGC 117.

During operation, the amplifier 202 amplifies the receivedcommunications signal 104 to generate an amplified signal 203. Theamplified signal 203 travels through the resistor 204, and is tapped offthe resistor 204 to the output 214 by a corresponding switch 208.Typically, only one switch 208 is closed at a time, so that only one tap206 is connected the common node 214. The tap 206 that is connected tothe common node 214 is referred to herein as the “selected tap”.

As such, the variable amplifier 210 provides a variable seriesresistance that attenuates the amplified signal 203, where theattenuation increases with increasing resistance. The resistance, andtherefore the attenuation, varies depending on which tap 206 isconnected the common node 214. The lowest resistance and attenuationoccur when the tap 206 a is the selected tap. The highest resistance andthe highest attenuation occur when the tap 206 n is the selected tap.The attenuation is increased by incrementally selecting taps in thedirection from 206 a to 206 n. Likewise, the attenuation is decreased byselecting taps in the direction of 206 n to 206 a.

For example, assume that switch 208 b is closed to select the tap 206 bas an initial condition. The attenuation can be increased relative tothe initial condition by opening switch 208 b and closing switch 208 cso as to select tap 206 c. The attenuation can be decreased relative tothe initial condition by opening the switch 208 b and closing the switch208 a to select the tap 204 a.

Typically, the PGA 200 is implemented on a integrated circuit (IC) wherethe circuit elements are deposited on the IC using known layout andprocessing techniques. Each switch 208 has a parasitic capacitance tothe IC ground, which causes an effective parasitic capacitance 216 toground at the common node 214, as shown in FIG. 2. The effectivecapacitance 216 limits the frequency bandwidth as will be understood bythose skilled in the arts. Further, the effective capacitance 216increases with the number of switches 208 (and therefore the number oftaps 206) because the switches 208 are in parallel, and parallelcapacitance is cumulative. Therefore, the frequency bandwidth of the PGA200 decreases as the number of taps 206 (and switches 208) increases. Asa result, there is trade-off between the granularity of the attenuation(i.e. number of taps) in the PGA 200, and the frequency bandwidth of thePGA 200.

3. PGA Description

FIG. 3A illustrates a PGA 300 according to one embodiment of the presentinvention. The PGA 300 includes the amplifier 202 and a variableresistor 301. Similar to the PGA 200, the variable resistor 301 includesa resistor 302 that connects the output of the amplifier 202 to groundor a reference voltage, and has multiple taps 304 that tap off theresistor 302 (e.g. resistor ladder). Additionally, the PGA 300 includesa two stage switch configuration that connects the taps 304 to an outputnode 310 of the PGA 300, instead of the single stage switchconfiguration in the PGA 200. More specifically, the taps 304 areconnected to the output node 310 by fine stage switches 306 and coarsestage switches 308. The taps 304 and corresponding fine stage switches306 are arranged into two or more groups 312, where each group 312 has agroup output terminal 307. One terminal of each fine stage switch 306 isconnected to the corresponding tap 304, and the other terminal isconnected to the group output terminal 307 for the corresponding group312. The output terminal 307 for each group 312 is connected to the PGAoutput node 310 by the corresponding coarse stage switch 308.

The nomenclature for the reference numbers in FIG. 3A is as follows. Thegroups 312 of switches 306 have been indexed from 1-to-n moving down thepage. For example, the first group is 312-1, the second group is 312-2,etc. The elements inside the groups 312 are given two index numbersafter the “-” represented here as “-ab”. The “a” represents the specificgroup 312 number in which the elements are located, and the “b”represents the element index within the group 312. For example, all theswitches 306 in group 312-1 are given a corresponding “-1” for the “a”index, and then numbered from 1-to-n for the “b” index. As a result, theswitches 306 in group 312-1 are referenced as 306-11, 306-12,306-13, . .. to 306-1 n. The switches 306 in group 312-2 are references as 306-21,306-22, 306-23 . . . 306-2 n. As will be apparent, there can be anynumber of switches 306 in a particular group 312, and any number ofgroups 312. A greater number of taps 304 permits smaller changes inincremental attenuation, as will be apparent to those skilled in thearts.

During operation, the amplifier 202 amplifies the receivedcommunications signal 104 to generate an amplified signal 203. Theamplified signal 203 travels through the resistor 302, and is tapped offthe resistor 302 at a selected tap 304 to the output node 310. Theamplified signal 203 is tapped off the resistor 302 by closing theappropriate switches 306 and 308. Therefore, the resistor 302 provides avariable series resistance that attenuates the amplified signal 203. Theamount of attenuation depends on which tap 304 is selected to beconnected to the output node 310 by the switches 306 and 308. A gaincontrol signal 303 determines the selected tap 304 by closing theappropriate fine stage switch 306 and coarse stage switch 308. Forexample, the gain control signal 303 can be an AGC signal, such as AGC117 in FIG. 1A.

Herein, the term “selected tap” will be used to refer to the tap 304that is connected to the output 310 by the switches 306 and 308.Similarly, the fine stage switch 306 that corresponds to the selectedtap 304 may be referred to as the “selected switch” 306. Similarly, thegroup 312 that contains the selected tap 304 and corresponding selectedswitch 306 may be referred to as the “selected group” 312.

One fine stage switch 306 and one coarse stage switch 308 are closed inorder to connect the selected tap 304 to the output node 310. Forexample, in order to select tap 304-11, then the fine stage switch306-11 and the coarse stage switch 308-1 are closed. In order to selecttap 304-23, the fine stage switch 306-23 and the coarse stage switch308-2 are closed. The lowest resistance, and therefore the lowestattenuation occurs when the tap 304-11 is the selected tap. The highestresistance, and therefore the highest attenuation, occurs when the tap304-nn is the selected tap. The attenuation is increased byincrementally selecting taps in the direction from 304-11 to 304-nn.Likewise, the attenuation is decreased by incrementally selecting tapsin the direction from 304-nn to 304-11. For example, if tap 304-12 isthe selected tap as an initial condition, then the attenuation can beincreased by changing the selected tap to tap 304-13. Likewise, theattenuation can be decreased by changing the selected tap to tap 304-11.

As in the conventional PGA 200, the switches 306 and 308 have aparasitic capacitance to ground that effects the frequency bandwidth ofthe PGA 300. The effective capacitance for each group 312 of switches306 is represented by capacitor 314 in FIG. 3B. The two stage switchconfiguration of the PGA 300 mitigates the effect of the groupcapacitances 314. This occurs because only the selected group 312 isconnected to the output node 310 by the corresponding (closed) switch308, and therefore only the parasitic capacitance 314 of the selectedgroup 312 is in the signal transmission path. The remaining non-selectedgroups 312 are isolated by the corresponding (open) switches 308. Forexample, if the tap 304 a is selected, then the switches 306 a and 308 aare closed. The remaining switches 308 are left open, and therefore onlythe effective parasitic capacitor 314 a of the group 312 a is connectedto the output 310. The remaining effective parasitic capacitors 314 areisolated from the output node 310 by their respective open switches 308.

The PGA 300 is illustrated as a singled-ended configuration. However,the PGA 300 can be configured as differential PGA, as will be understoodby those skilled in the arts.

4. Transient Voltage Considerations

Transient voltages can be created when the tap selection is changed tovary the attenuation of the PGA 300. The transient voltage occursbecause the parasitic capacitances associated with switches 306 and 308store and release energy when the switches are closed and opened. Forexample, if the tap selection is changed from 304-1 n (in group 312-1)to tap 304-21 (in group 312-2), then the switches 306-1 n and 308-1 areopened, and the switches 306-21 and 308-2 are closed. When the switch306-1 n is opened, charge that was stored on the parasitic capacitanceofthe switch 306-1 n is discharged. Likewise, when the switch 308-2 isclosed, charge is transferred and stored on the parasitic capacitance ofthe switches 306-21 until the parasitic capacitance is fully charged.The capacitor charging and discharging operations produce a transientvoltage that appears at the output node 310 of the PGA 300. If thetransient voltage does not settle quickly enough then it can cause falseerrors during the CRC calculations that are performed by the DSP 112during demodulation. Therefore, it is preferable to minimize the effectsof the transient voltages by settling the transient voltages as quicklyas possible.

The settling time of the transient voltage can be reduced by closingadditional fine stage switches 306, beyond the particular fine stageswitch 306 that corresponds to the selected tap 304. By judiciallyclosing switches 306 in non-selected groups 312, the parasiticcapacitance for the fine stage switches 306 is pre-charged, therebyreducing the settling time of the transient voltage that accompanies achange in gain settings. The following sections describe two suchconfigurations that reduce the transient voltage settling time byclosing the additional fine stage switches 306 in non-selected groups312.

5. Turn-On at Least One Switch in Each Group

FIGS. 4A-4B illustrate one embodiment for reducing transient voltagesettling time by closing additional switches 306 in non-selected groups312. In this embodiment, at least one switch 306 is closed in each group312, even in those groups 312 that do not have the selected tap 304. Theswitches 306 that are closed in the non-selected groups 312 have thesame corresponding location (or index) as for the selected tap 304. Thefollowing examples further illustrate the switches 306 that are closedin the non-selected groups 312.

For example, in FIG. 4A, the tap 304-11 is the selected tap in theselected group 312-1, and therefore switches 306-11 and 308-1 areclosed. Additionally, the following fine stages switches 304 in thenon-selected groups 312 are also closed: switch 306-21 (in group 312-2),switch 306-31 (in group 312-3), and switch 306-n 1 (in group 312 n),etc. Therefore, at least one switch 306 in each group 312 is closed atall times, which pre-charges the parasitic capacitance ofthe switches306 in each group 312 by some amount. By pre-charging the parasiticcapacitances, the transient voltage is reduced when the tap selection ischanged to a new group 312. The coarse stage switches 308-2, 308-3, and308-n for the corresponding non-selected groups 312-2,312-3 and 312-nare left open, thereby isolating the corresponding fine stage switches306 in these groups from the output 310.

The closed switches 306 in the non-selected groups 308 have the samelocation (or “index”) within the group 312 as for the selected switch306-11 in the selected group 312-1. In other words, the selected tap304-11 is the first tap in the group 312, and the corresponding switch306 is the first switch in the group 312. Likewise, the closed switches306-21, 306-31, and 306-n 1 are also the first switches in theirrespective groups 312.

FIG. 4B illustrates a second example for this embodiment, where the tap304-22 is the selected tap in the selected group 312-2. The switches306-22 and 308-2 are closed to connect the selected tap 304-22 to theoutput 310. Additionally, the following fine stage switches in thenon-selected groups 312 are also closed: switch 306-12 (in group 312-1),switch 306-32 (in group 312-3), and switch 306-n 2 (in group 312-n),etc. The corresponding coarse stage switches 308-1, 308-3, and 308-n areleft open.

6. Turn-On Switches in Each Group in a Mirror Image Order

In a second embodiment, some of the closed switches 306 in non-selectedgroups 312 have a different relative location when compared to thelocation of the selected tap 304. More specifically, the location oftheclosed switches 306 in the non-selected groups is the mirror image ofthe location in an adjacent group 312.

FIGS. 5A-5E further illustrate the location of the closed switches 306in the non-selected groups 312 according to this mirror imageembodiment. FIG. 5A illustrates an initial switch configuration for aninitial attenuation setting. FIGS. 5B-5E illustrate the progression ofswitch configurations for increased attenuation and the switch operationin non-selected groups 312. As in prior sections, the switches 306 inthe non-selected groups 312 are closed to pre-charge the parasiticcapacitance that is associated with the switches 306 and 308.

In FIGS. 5A-5E, it is noted that the number of switches 306 in eachgroup 312 is set to n=4 for ease of discussion. As will be apparent,each group 312 could contain any number of switches 306. Furthermore, inFIGS. 5A-5E, the fine stage switches 306 are arranged into five groups312 (312-1 to 312-5). As will be apparent, the fine stage switches 306can be arranged into any number of groups.

In FIG. 5A, the tap 304-11 is the selected tap in the selected group312-1. The tap 304-11 is at the absolute top of the resistor 302 so thesignal attenuation to the output node 310 is a minimum. The switches306-11 and 308-1 are closed to connect the selected tap 304-11 to theoutput 310. Additionally, the switches 306-24, 306-31, 306-44, and306-51 in the corresponding non-selected groups 312-2 to 312-5 are alsoclosed, so as to pre-charge the associated parasitic capacitance 314 forthe corresponding non-selected groups.

It is noted that the locations of the switches 306 that are closedvaries from over the groups 312. More specifically, the closed switches306 in adjacent groups 312 are at mirror image locations about theboundary between the groups 312. For example, the selected switch 306-11in FIG. 5A is the first switch in the group 312-1, and the switch 306-24is the last switch in the group 312-2, which is the mirror image of theswitch 306-11 about a boundary 316-1 between the groups 312-1 and 312-2.The switch 306-31 is the first switch in the group 312-3, which is themirror image of the switch 306-24 in group 312-2 about a boundary 316-2between the group 312-2 and 312-3. The switch 306-44 is the last switchin the group 312-4, which is the mirror image of the switch 306-31 ingroup 312-3 about a boundary 316-3 between the groups 312-3 and 312-4.The switch 306-51 is the first switch in the group 312-5, which is themirror image of the switch 306-44 in the group 312-4 about a boundary316-4 between the groups 312-4 and 312-5.

In FIG. 5B, tap 304-12 is the selected tap, and therefore the switches306-12 and 308-1 are closed to connect the selected tap 304-12 to theoutput 310. Additionally, the switches 306-23, 306-32, 306-43, and306-52 are closed in the corresponding non-selected groups 312-2 to312-5, so as to pre-charge the parasitic capacitances of the switches306 in these non-selected groups.

As in FIG. 5A, the closed switches 306 in adjacent groups 312 are atmirror image locations about the boundary between the adjacent groups312. For example, the selected switch 306-12 is the second switch in thegroup 312-1, and the switch 306-23 is the third switch in the group312-2, which is the mirror image of the selected switch 306-12 about theboundary 316-1. The switch 306-32 is the second switch in the group312-3, which is the mirror image of the switch 306-23 in group 312-2about the boundary 316-2. The switch 306-43 is the third switch in thegroup 312-4, which is the mirror image of the switch 306-32 in group312-3 about the boundary 316-3. The switch 306-52 is the second switchin the group 312-5, which is the mirror image of the switch 306-43 inthe group 312-4 about the boundary 316-4.

In FIG. 5C, tap 304-13 is the selected tap, and therefore the switches306-13 and 308-1 are closed to connect the selected tap 304-13 to theoutput 310. Additionally, the switches 306-22, 306-33, 306-42, and306-53 in the corresponding non-selected groups 312-2 to 312-5 are alsoclosed, so as to pre-charge the parasitic capacitances of the switches306 in the non-selected groups 312-2 to 312-5.

As in FIGS. 5A-5B, the closed switches 306 in adjacent groups 312 inFIG. 5C are at mirror image locations about the boundary between theadjacent groups 312. For example, the selected switch 306-13 is thethird switch in the group 312-1, and the switch 306-22 is the secondswitch in the group 312-2, which is the mirror image of the switch306-13 in group 312-1 about the boundary 316-1. The switch 306-33 is thethird switch in the group 312-3, which is the mirror image of the switch306-22 in the group 312-2 about the boundary 316-2. The switch 306-42 isthe second switch in the group 312-4, which is the mirror image of theswitch 306-33 in the group 312-3 about the boundary 316-3. The switch306-53 is the third switch in the group 312-5, which is the mirror imageof the switch 306-42 in the group 312-4 about the boundary 316-4.

In FIG. 5D, the tap 304-14 is the selected tap, and therefore theswitches 306-14 and 308-1 are closed to connect the selected tap 304-14to the output 310. Additionally, the switches 306-21, 306-34, 306-41,and 306-54 in the corresponding non-selected groups 312-2 to 312-5 arealso closed, so as to pre-charge the parasitic capacitances of theswitches 306 in the non-selected groups 312-2 to 312-5.

As in FIGS. 5A-5C, the closed switches 306 in adjacent groups 312 are atmirror image locations about the boundary between the adjacent groups312. For example, the selected switch 306-14 is the last switch in thegroup 312-1, and the switch 306-21 is the first switch in the group312-2, which is the mirror image of the switch 306-14 in group 312-1about the boundary 316-1. The switch 306-34 is the fourth switch in thegroup 312-3, which is the mirror image of the switch 306-21 in group312-2 about the boundary 316-2. The switch 306-41 is the first switch inthe group 312-4, which is the mirror image of the switch 306-34 in thegroup 312-3 about the boundary 316-3. The switch 306-54 is the lastswitch in the group 312-5, which is the mirror image of the switch306-41 in the group 312-4 about the boundary 316-4.

In FIG. 5E, tap 304-21 is the selected tap, and therefore the switches306-21 and 308-2 are closed to connect the selected tap 304-21 to theoutput 310. It is noted that switch 306-21 is already closed because ofthe mirror image switch closing process for non-selected groups 312 thatis illustrated by FIGS. 5A-5D. Since switch 306-21 is already closed,the parasitic capacitance that is associated with the switch 306-21 andthe group 312-2 is already charged-up. This significantly reduces thetransient voltage that is normally associated with tap changes, andimproves the settling time for any transient voltage that remains. Forexample, in embodiments, the transient voltage is reduced from 100 mv toas low as 10 mV.

As stated above, the closed switches 306 in adjacent groups 312 are atmirror image locations about the boundary between the adjacent groups312. The position ofthe closed switches 306 can be described in anequivalent but different manner. To preface this discussion, it is notedthat the groups 312 are indexed from 1-to-n (e.g. 312-1, 312-2, etc.)Hence, there are even numbered groups 312 (e.g. 312-2,312-4) and oddnumbered groups 312 (e.g. 312-1,312-3,312-5) For convenience, it isassume that the selected switch 306 is the m^(th) switch (out of n) in aselected group 312. If the selected switch 306 is located in an evennumbered group 312 (e.g. 312-2, 312-4, etc.), then the m^(th) switch isclosed in all the even numbered groups 312. Additionally, the[(n+1)−m^(th)] switch 306 is closed in all the odd numbered groups 312.Similarly, if the selected switch 306 is located in an odd numberedgroup 312 (e.g. 312-1, 312-3, etc.), then the m^(th) switch 306 isclosed in all the odd numbered groups 312, and the [(n+1)−m^(th)] isclosed in the even numbered groups 312.

As an example, in FIG. 5A, the tap 304-11 is the selected tap so thatthe switches 306-11 and 308-1 are closed to connect the tap 304-11 tothe output 310. The switch 306-11 is the first switch in the group312-1, which is an odd numbered group. In accordance with the discussionabove, the first switches 306 in the odd numbered groups 312 are to beclosed. This is born out in FIG. 5A as switches 306-31 and 306-51 areclosed in the odd numbered groups 312-3 and 312-5, respectively.Additionally, the (n+1)−m^(th) switches are to be closed in the evennumbered groups according to the discussion above. Since n=4 (as thereare 4 switches in each group 312) and m=1 (as the first switch 306-11corresponds to the selected tap 304-11) , then:(n+1)−m=(4+1)−1=4

Therefore, the 4th switch in the even numbered groups 312 is to beclosed. This is born out in FIG. 5A as switches 306-24 and 306-44 areclosed the groups 312-2 and 312-4, respectively. Note that switches306-24 and 306-44 are the fourth switches in FIGS. 5A-5E.

As a second example, in FIG. 5B, the tap 304-12 is the selected tap sothat the switches 306-12 and 308-1 are closed to connect tap 304-12 tothe output 310. The switch 306-12 is the second switch in the group312-1, which is an odd numbered group. In accordance with the discussionabove, the second switch 306 in each odd numbered group 312 is to beclosed. This is born out in FIG. 5B as switches 306-32 and 306-52 areclosed in the odd numbered groups 312-3 and 312-5, respectively.Additionally, the [(n+1)−m]^(th) switch is to be closed in each of theeven numbered groups. Since n=4 and m=2, then:(n+1)−m=(4+1)−2=3

Therefore, the 3rd switch in the even numbered groups 312 is to beclosed. This is born out in FIG. 5B as switches 306-23 and 306-43 areclosed the groups 312-2 and 312-4, respectively.

The operation of the PGA 300 is further described according to flowchart700 that is shown in FIG. 7, which is described as follows.

In step 702, a gain control signal is received that determines theattenuation of the variable resistor 301, and therefore the gain of thePGA 300. The gain control signal identifies the selected tap 304 that isto be connected to the output 310. For example, the gain control signalcan be an automatic gain control (AGC) signal, such as AGC signal 117(FIG. 1) that is generated by the AGC module 116.

In step 704, the fine stage switch 306 and the coarse stage switch 308that correspond to the selected tap 304 are closed. The fine stageswitch 306 that corresponds to the selected tap 304 is identified as them^(th) switch 306 (out of n) in the selected group 312. For example, inFIG. 5A, tap 304-11 is the selected tap so that the fine stage switch306-11 and the coarse stage switch 308-1 are closed to connect theselected tap 304-11 to the output 310. The switch 306-11 is the firstswitch (out of 4) in the selected group 312-1.

In step 706, the determination is made as to whether the selected tap304 and corresponding switch 306 are in an even numbered group 312 or anodd numbered group 312. If the selected tap 304 is in an even numberedgroup 312, then control flows to step 708. If the selected tap 304 is inan odd numbered group 312, then control flows to step 712. For example,in FIG. 5A, the selected tap 304-1 is in group 312-1, which is an oddnumbered group.

In step 708, the selected tap 304 is in an even numbered group,therefore the m^(th) switch 306 is closed in each even numbered group312 that is a non-selected group 312 (Note that the switch correspondingto the selected tap 304 was closed in step 704). Additionally, in step710, the [(n+1)−m^(th)] switch 306 is closed in every odd numbered group312.

In step 712, the selected tap 304 is in an odd numbered group, thereforethe m^(th) switch 306 is closed in every odd numbered group 312 that isa non-selected group 312 (Note that the switch 306 corresponding to theselected tap 304 was closed in step 704). Additionally, in step 714, the[(n+1)−m^(th)] switch 306 is closed in every even numbered group 312.For example, in FIG. 5A, switches 306-31 and 306-51 are closed inadditional to switch 306-11.

In step 716, the flowchart ends.

7. Transmission Line Characteristics of 2-Stage Switch Configuration

A further benefit of the PGA 300 with the 2-stage switch configurationis that the overall input impedance of the variable resistor 301 iscloser to that of a transmission line. Referring to FIG. 3B, theresistor 302 and the parallel effective capacitors 314 have adistributed characteristic that closely approximates the impedance of atransmission line, for example a cable. As a result, the 3 dB cutofffrequency substantially matches that of transmission line, asillustrated by curve 602 in FIG. 6.

The input impedance of PGA 300 appears as a distributed RC networkbecause the resistance and capacitance ofthe PGA 300 are distributedthrough the two stages. As a result, the PGA 300 has an amplituderoll-off that varies as 1/{square root}{square root over (freq)}.Furthermore, in one embodiment, there is an inverse relationship betweenthe PGA tap selection and the cable length (i.e. cable 102). Forexample, given a relatively short cable, tap 304-nn (FIG. 3A) can beselected to set a relatively high attenuation for the PGA 300. Given arelatively long cable, the tap 304-11 can be selected to set arelatively low attenuation for the PGA 300. By using this inverserelationship, less equalization is needed for the DSP 112.

8. Multi-Stage Configurations

As described herein, the PGA 300 is a two-stage PGA. However, theinvention is not limited to a two-stage PGA, as the present inventioncan be implemented in a multistage PGA having more than two stages. Inother words, the switching configurations and methods described herein,can be implemented in a multi-stage PGA, as will be understood by thoseskilled in the arts based on the teachings given herein.

9. Other Applications

The PGA invention described herein has been discussed in reference to areceiver. However, the PGA is not limited to receivers, and isapplicable to other non-receiver applications that benefit from lowtransient voltages and good frequency bandwidth. The application ofthePGA invention to these non-receiver applications will be understood bythose skilled in the relevant arts based on the discussions givenherein, and are within the scope and spirit of the present invention.

10. Conclusion

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any ofthe above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1-19. (Canceled).
 20. A programmable gain amplifier (PGA), comprising: aresistor having first adjacent taps and second adjacent taps; firstswitching means for switchably coupling the first adjacent taps to afirst output terminal; second switching means for switchably couplingthe second adjacent taps to a second output terminal; and thirdswitching means for switchably coupling the first output terminal andthe second output terminal to a third output terminal.
 21. The PGA ofclaim 20, further comprising an amplifier having an output coupled to aninput of the resistor.
 22. The PGA of claim 21, wherein the resistor,the first switching means, the second switching means, the thirdswitching means, and the amplifier have a common substrate.
 23. The PGAof claim 20, wherein the resistor is connected to a reference voltage.24. The PGA of claim 20, wherein if the first switching meanselectrically couples a tap of the first adjacent taps to the firstoutput terminal, then the second switching means electrically couples atap of the second adjacent taps to the second output terminal.
 25. ThePGA of claim 20, wherein if an nth switch of the first switching meansis closed, then an n^(th) switch of the second switching means isclosed.
 26. The PGA of claim 20, wherein if an nth switch of the firstswitching means is closed, then an [(m+1)−n^(th]) switch of the secondswitching means is closed, and wherein m is a number of switches in thefirst or second switching means.
 27. The PGA of claim 20, wherein thethird switching means electrically couples the first output terminal tothe third output terminal, and the first switching means electricallycouples a tap of the first adjacent taps to the first output terminal,and the second switching means electrically couples a tap of the secondadjacent taps to the second output terminal to pre-charge a parasiticcapacitance of the second switching means.
 28. The PGA of claim 20,wherein the resistor, the first switching means, the second switchingmeans, and the third switching means have a common substrate.
 29. ThePGA of claim 28, wherein said common substrate is a CMOS substrate. 30.A programmable gain amplifier (PGA), comprising: a resistor having afirst plurality of adjacent taps and a second plurality of adjacenttaps; a first plurality of switches having input terminals correspondingto the first plurality of adjacent taps and having a first outputterminal; a second plurality of switches having input terminalscorresponding to the second plurality of adjacent taps and having asecond output terminal; and switching means for switchably coupling thefirst output terminal and the second output terminal to a third outputterminal.
 31. The PGA of claim 30, further comprising an amplifierhaving an output coupled to an input of the resistor.
 32. The PGA ofclaim 31, wherein the resistor, the first plurality of switches, thesecond plurality of switches, the switching means, and the amplifierhave a common substrate.
 33. The PGA of claim 30, wherein the resistoris connected to a reference voltage.
 34. The PGA of claim 30, wherein ifa switch of the first plurality of switches electrically couples a tapof the first plurality of adjacent taps to the first output, then the aswitch of the second plurality of switches electrically couples a tap ofthe second plurality of adjacent taps to the second output.
 35. The PGAof claim 30, wherein if an n^(th) switch of the first plurality ofswitches is closed, then an n^(th) switch of the second plurality ofswitches is closed.
 36. The PGA of claim 30, wherein if an n^(th) switchof the first plurality of switches is closed, then an [(m+1)−n^(th])switch of the second plurality of switches is closed, and wherein m is anumber of switches in the first or second plurality of switches.
 37. ThePGA of claim 30, wherein the switching means electrically couples thefirst output terminal to the third output terminal, and a switch of thefirst plurality of switches electrically couples a tap of the firstplurality of adjacent taps to the first output terminal, and a switch ofthe second plurality of switches electrically couples a tap of thesecond plurality of adjacent taps to the second output terminal topre-charge a parasitic capacitance of the switch of the second pluralityof switches.
 38. The PGA of claim 30, wherein the resistor, the firstplurality of switches, the second plurality of switches, and theswitching means have a common substrate.
 39. The PGA of claim 38,wherein said common substrate is a CMOS substrate.